Fabrication of magnetoresistive random-access memory (MRAM) devices normally involves a sequence of processing steps during which many layers of metals and dielectrics are deposited and then patterned to form a magnetoresistive stack as well as electrodes for electrical connections. To define the magnetic tunnel junctions (MTJ) in each MRAM device, precise patterning steps including photolithography and reactive ion etching (RIE), ion beam etching (IBE), or their combination are usually involved. MTJ critical dimension (CD) variation is a critical parameter because large CD variation can cause significant variations in MTJ resistance and switching current. Besides pattern density, the CD variation is mainly dependent on photo CD variation, which is limited mainly by wavelength of the exposure light, dose, focus, lens aberration, and mask error. The MTJ area may vary as much as 10% due to photoresist sidewall roughness. However, larger photo CD will always have less variation than smaller photo CD due to less wavelength effect and/or less edge effect. Sidewall roughness leads to large CD variation for small CD. For example, if the sidewall roughness is 1.5 nm, it will be 3 nm on both sides. This will cause a 2% CD variation for a 150 nm device, but a 4% CD variation for a 75 nm device.
Using short wavelength light for exposure is the most effective way to make small CD with better CD uniformity. The dominant lithography technology uses 248 nm or 193 nm wavelength; however, the current state-of-the-art photolithography is 13.5 nm EUV. It desired to find a way to minimize photoresist roughness and CD variation.
Several patents teach methods of hard mask trimming, including U.S. Pat. No. 9,779,952 (Ranjan et al), U.S. Pat. No. 7,910,483 (Abatchev et al), and U.S. Pat. No. 7,550,044 (Gaidis et al) and U.S. Patent Application 2016/0268338 (Kashiwada et al). These references are different from the present disclosure.